The present invention relates to a planar lighting device used for a liquid crystal display device and the like.
Liquid crystal display devices use a backlight unit for radiating light from behind the liquid crystal display panel to illuminate the liquid crystal display panel. A backlight unit is configured using a light guide plate for diffusing light emitted by an illuminating light source to irradiate the liquid crystal display panel and optical parts such as a prism sheet or a diffusion sheet for rendering the light emitted from the light guide plate uniform.
Currently, large liquid crystal televisions predominantly use a so-called direct illumination type backlight unit comprising a light guide plate disposed immediately above the illuminating light source. This type of backlight unit comprises a plurality of cold cathode tubes serving as a light source provided behind the liquid crystal display panel whereas the inside of the backlight unit provides white reflection surfaces to ensure uniform light amount distribution and necessary brightness.
To achieve a uniform light amount distribution with a direct illumination type backlight unit, however, a thickness of about 30 mm in a direction perpendicular to the liquid crystal display panel is required, making further reduction of thickness difficult with the direct illumination type backlight unit.
Among backlight units that allow reduction of thickness thereof is a backlight unit using a light guide plate whereby light emitted by illumination light sources and admitted into the light guide plate is guided in given directions and emitted through a light exit plane that is different from the plane through which light is admitted.
There has been proposed a backlight of a type described above using a light guide plate formed by mixing scattering particles for diffusing light into a transparent resin, as follows:
JP 07-36037 A, for example, discloses a light diffusion/guide type light source device comprising a light diffusion/guide member having at least one light entrance plane region and at least one light exit plane region and light source means for admitting light through the light entrance plane region, the light diffusion/guide member having a region that has a tendency to decrease in thickness with the increasing distance from the light entrance plane.
JP 08-248233 A discloses a planar light source device comprising a light diffusion/guide member, a prism sheet provided on the side of the light diffusion/guide member closer to a light exit plane, and a reflector provided on the rear side of the light diffusion/guide member. JP 08-271739 A discloses a liquid crystal display comprising a light emission direction correcting element formed of a sheet optical member provided with a light entrance plane having a repeated undulate pattern of prism arrays and a light exit plane given a light diffusing property. JP 11-153963 A discloses a light source device comprising a light diffusion/guide member having a scattering property therein and light supply means for supplying light through an end face of the light diffusion/guide member.
Also proposed in addition to the above light guide plates are a light guide plate having a greater thickness at the center thereof than at an end thereof at which light is admitted and the opposite end, a light guide plate having a reflection plane inclined in such a direction that the thickness of the light guide plate increases with the increasing distance from a part of the light guide plate at which light is admitted, and a light guide plate having a configuration such that a thickness thereof is minimum at a light entrance portion and is greatest at a greatest distance from the light entrance portion (See, for example, JP 2003-90919 A, JP 2004-171948 A, JP 2005-108676 A, and JP 2005-302322 A).
While a thin design may be achieved with backlight units such as the tandem type, of which the thickness decreases with the increasing distance from the light source, those backlight units yielded lower light use efficiency than the direct illumination type because of the relative dimensions of the cold cathode tube to the reflector. Further, where a light guide plate used is shaped to have grooves for receiving cold cathode tubes, although such a light guide plate could be shaped to have a thickness that decreases with the increasing distance from the cold cathode tubes, if the light guide plate is made thinner, brightness at locations immediately above the cold cathode tubes disposed in the grooves increased, thus causing uneven brightness on the light exit plane to stand out. In addition, all these light guide plates posed further problems: complex configurations leading to increased machining costs. Thus, a light guide plate of any of such types adapted to be used for a backlight unit for a large liquid crystal television having a screen size of say 37 inches or larger, in particular 50 inches or larger, was considerably expensive.
In each light guide plate proposed in JP 2003-90919 A, JP 2004-171948 A, JP 2005-108676 A, and JP 2005-302322, a thickness of the light guide section grows greater with the increasing distance from the light entrance plane in order to achieve stabler manufacturing or to suppress brightness unevenness (unevenness in light amount) using multiple reflection. Each of these light guide plates, however, is made of a transparent material, and allows light admitted from the light source to pass and leak through the opposite end and therefore need to be provided with prisms or dot patterns on the surface thereof.
Also proposed is a method whereby the light guide plate is provided with a reflection member on the opposite side to the light entrance plane to cause admitted light to undergo multiple reflection before allowing the light to exit through the light exit plane. To achieve a large light exit plane with the light guide plate by this method, however, the light guide plate needs to have an increased thickness, increasing weight and costs. Further, the light sources are projected into the light guide plate and perceived as such to cause uneven brightness.
On the other hand, the side light type backlight unit using a flat light guide plate contains fine scattering particles dispersed therein in order to efficiently emit admitted light through the light exit plane. Although such a flat light guide plate may be capable of securing a light use efficiency of 83% at a particle density of 0.30 wt %, its brightness becomes dropped in an area about the center of the light guide exit plane as illustrated by the illuminance distribution indicated by a solid line in FIG. 19 when it was adapted to provide a larger screen despite scattering particles evenly dispersed therein, thus allowing uneven brightness to stand out to a visible level.
To even out such uneven brightness, the density of the scattering particles needed to be reduced in order to increase the amount of light leaking from the forward end, thus reducing the light use efficiency and the brightness. For example, when the density of the scattering particles was 0.10 wt %, with the other conditions being equal, the brightness decreased and the light use efficiency lowered to 43%, although uneven brightness could be evened out considerably, as illustrated by a dotted line in FIG. 19.
A large display such as a large liquid crystal television requires a brightness distribution that is bright at a central area of the screen as compared with the periphery thereof, or a high-in-the-middle distribution such as a distribution representing a bell curve on the light exit plane. Although a flat light guide plate containing scattering particles dispersed therein may be capable of providing a flat brightness distribution by reducing the density of the scattering particles, it is incapable of achieving a high-in-the-middle brightness distribution.
It has also been proposed to use a light guide plate having a thickness that, conversely to the tandem type, increases with the increasing distance from the light source for a thin backlight unit. Although use of such a light guide plate does achieve a thinner design and a flat brightness over the whole screen, such a proposal did not provide any teaching or did not give the slightest consideration as to how one may achieve a high-in-the-middle brightness distribution whereby an area close to the center of the screen is brighter than the periphery thereof as required of thin, large-screen liquid crystal televisions.
Further, a large light guide plate expands and contracts greatly in response to ambient temperature and humidity changes; a light guide plate measuring about 50 inches, for example, repeats expansion and contraction of 5 mm or more. In worst cases, an expanded or contracted light guide plate pushes up the liquid crystal display panel, causing uneven brightness comparable in appearance to a pool. To avoid this, one might consider providing a great distance between the liquid crystal display panel and the backlight unit. This approach, however, makes it impossible to design a thinner liquid crystal display device.